This invention relates to a hydraulic switch mechanism for an automotive vehicle power steering device.
A related-art hydraulic switch mechanism for a power steering device is shown in FIGS. 5 through 7. In a hydraulic pressure switch mechanism 50, an input shaft 51 is arranged concentrically with an output shaft 52 having a cylindrical shape. The input shaft 51 comprises a small-diameter cylindrical portion 51a and a large-diameter cylindrical portion 51b. An end of the small-diameter cylindrical portion 51a is connected to the large diameter cylindrical portion 51b, and the other end of the small-diameter cylindrical portion 51a is geared to a steering shaft (not shown). The output shaft 52 has on its outer periphery a pinion 53 which meshes with a rack 54 for driving vehicle wheels. As shown in FIG. 5, the upper portion of the output shaft 52 above the pinion 53 is inserted around the large-diameter cylindrical portion 51b of the input shaft 51 via an elastic pre-loaded spring ring 55 (hereinafter referred to as the C ring). In a cutout in the C ring 55, a drive pin 56 on the large-diameter cylindrical portion 51b engages an engaging portion 52a of the output shaft 52. A valve inner spool 57 loosely extends through the output shaft 52. The upper portion of the valve inner spool 57 is attached via a drive pin 58 to the small-diameter cylindrical portion 51a of the input shaft 51. The lower portion of the valve inner spool 57 extends downward from the output shaft 52. The valve inner spool 57 includes a projection 57a having a square cross section. A hydraulic passage 59 is provided between opposite walls of the projection 57a. Each corner of the projection 57a is chamfered to form a sealing portion 60. A valve outer spool 61 surrounds the projection 57a and engages the output shaft 52 via a drive pin 63. As shown in FIG. 7, when an automotive vehicle drives straight ahead and the input shaft 51 as well as the valve inner spool 57 do not rotate, four hydraulic passages 61a are connected to a hydraulic cylinder (not shown) for axially moving the rack 54 and two hydraulic passages 61b are connected to a hydraulic pump (not shown). A valve casing 62 includes a hydraulic groove 62a connected to the hydraulic pump.
In operation, when the automotive vehicle turns, the projection 57a of the valve inner spool 57 rotates relative to the valve outer spool 61, and the sealing portion 60 contacts an inner periphery of the valve outer spool 61 for oil sealing. When the automotive vehicle is steered and the input shaft 51 rotates relative to the output shaft 52, the C ring 55 generates a torque that acts to produce rotation in a steering wheel.
The hydraulic switch mechanism 50 has the following problems.
(1) The projection 57a of the valve inner spool 57 and the valve outer spool 61 are arranged below the rack 54 and opposite to the steering shaft. The hydraulic switch mechanism 50 has such a large size and complicated structure.
(2) Since the valve inner spool 57 extends through the output shaft 52, the pinion 53 must be thick, thereby increasing the weight of the hydraulic switch mechanism 50.